Abstract

Magnetotelluric data from four profiles crossing the Eocene strike-slip Fraser River fault in southwestern British Columbia suggest that it penetrates the entire crust. This conclusion is supported by seismic reflection observations of a 2-3 km step in the Moho slightly to the east of the surface expression of the fault, but is at variance with an interpretation of the seismic data in which the fault soles into mid-crustal reflectors that seem to be continuous across the fault trace. A crustal-penetrating geometry supports the proposal that the Fraser River fault forms part of a 2500-km-long intracontinental transform fault system in northwestern North America. Modeling studies resolve a thin, highly conducting mid-crustal zone that is connected electrically to the conducting lower crust beneath the Coast belt. Low δ13C values close to the Fraser fault system suggest that the electromagnetic signature of this zone may be due to the presence of organic carbon.